Recently, in the molding of optical elements such as optical glass lenses employed in optical devices such as cameras and pickups, numerous methods of manufacturing optical glass elements by press molding a heat-softened optical glass material in a mold made of metal, ceramic, or the like without processing such as polishing have been proposed and put into practice.
However, since the press-molded lens shrinks in the cooling step following pressing in this technique, it is impossible to obtain a lens to which the shape of the molding surface of the mold has been precisely transferred. Accordingly, a method comprising preparing a mold that has been processed into a shape canceling out the error in the shape of the lens relative to the molding surface and using it for pressing has been proposed.
For example, Unexamined Japanese Patent Publication (KOKAI) Heisei Nos. 6-72726 and 8-337426 disclose methods of press molding a glass material with a mold that has been preprocessed to a shape canceling out constant anomalies occurring in the lens due to shrinkage caused by cooling following press molding.
However, methods such as the above employing a mold that has been processed to a shape canceling out the error in shape of the lens present the following problems.
In methods of correcting the shape of the mold, the molding surface changes over time with the number of pressings even under stable pressing conditions, sometimes causing the shape of the mold to change slightly over time. Accordingly, an error in the shape of the lens occurs, and each time the permitted range is exceeded, successive mold-processing steps become necessary, with pressing being temporarily stopped during these mold-processing steps. As a result, there are problems in that the lead time from product order to product completion increases, production efficiency decreases, and cost increases.
Conventionally, so long as strict optical characteristics are not required, production has been continued and the change over time in the molding surface due to the accumulated number of pressings has been ignored even when there has been some error in lens shape. It has not been possible to employ methods of correcting the mold shape to handle such cases.
However, extremely precise optical characteristics are required of today's lenses, such as optical pickup lenses and image pickup lenses.
For example, high-density recording is conducted with high NA objective lenses for optical pickups. As a result, the diameter of the beam spot on the light focused on the optical disk has decreased. As this has occurred, reducing the amount of aberration generated by position error and tilt during lens installation and lens driving have become major technical issues. There is a need to improve the properties of the lens itself (reduce wave front aberration) prior to installation.
Improving the properties of the lens itself requires designing lenses with little wave front aberration and reducing lens manufacturing errors in the designed values of lenses. However, as the lens NA has increased and the wavelengths employed have shortened, the amount of aberration permitted due to manufacturing error, which is inversely proportional to the 2nd or 3rd power of the NA and inversely proportional to wavelength, has decreased (requirements have become stricter). Thus, it is actually quite difficult to stably manufacture high-performance lenses.
In converging optical systems for optical pickups, it is possible to achieve properties with configurations comprising multiple lenses and multiple lens groups. When this is done, although it is possible to relax the error permitted in the manufacturing of individual lenses, the increase in the number of lenses creates new problems in areas such as size reduction and adjusting the positioning between lenses. Accordingly, in converging optical systems for optical pickups, an individual lens (single lens) is required to have a high NA. However, the permissible error in manufacturing a single lens is smaller than that permitted to multiple lenses and multiple lens groups. For example, the permissible error for a single lens with an NA of 0.85 is 3 to 10 times stricter than for two groups of two lenses.
Further, while there is an upper limit to the refractive index in the lens material, the designing of high-performance high-NA lenses requires that the maximum surface inclination angle (the maximum angle formed between the normal at any point on a lens and the optical axis) of at least one surface of the lens be increased. To achieve reduction in size requires that the effective diameter of the lens and the lens outer diameter be reduced. These are also factors that have resulted in strict levels of permissible error in manufacturing.
The wave front aberration of a lens is comprised chiefly of spherical aberration, coma aberration, and astigmatism. In lens design, an optimal design is sought in which axial wave front aberration and spherical aberration are as close to zero as possible (since coma aberration and astigmatism result from manufacturing errors in lens surface inclination, eccentricity, and symmetry, they are naturally zero during the design stage).
Press molding of lenses having a wave front aberration of less than or equal to 0.04 λrms during manufacturing requires first that spherical aberration, particularly third-order spherical aberration, be as low as possible: within ±0.02 λrms, preferably within ±0.01 λrms.
Spherical surface aberration is caused by manufacturing errors such as error in the radius of curvature of the lens surface (in the case of an aspherical surface, the paraxial radius of curvature) error in lens surface shape, error in lens thickness, and error in the refractive index of the lens material. In prior art mold processing techniques and pressing techniques, it is extremely difficult to precisely control and maintain stable precision of factors in lens spherical aberration such as surface shape precision and thickness precision. In particular, the generation of change in spherical aberration accompanying change in the surface state (a tribological characteristic of the mold separation film and glass) of the mold affecting mold transfer precision during continuous pressing, and ways of preventing it, have been completely unknown.
Mold shape precision and thickness control precision must be kept to within critical values of mechanical precision in mold processing machinery and press-molding machinery to reduce spherical aberration and control variation to within desired values. Accordingly, it is thought that the mass production of such lenses of high molding difficulty is practically impossible by controlling mold shape precision and thickness control precision.
In particular, in optical pickups employing a blue violet laser, it is necessary to reduce the spherical aberration of the entire optical system. Thus, an optical element is employed to correct the overall spherical aberration produced by installation error in the objective lens and other optical elements and positional and angular shift during driving.
However, when the amount of correction is large, the fifth-order spherical aberration increases markedly when third-order spherical aberration is reduced. As a result, wave front aberration increases, compromising the quality of the converged beam spot. Thus, the amount of aberration corrected for with the correcting optical element is desirably kept as low as possible.
According, it is first highly desirable to keep the spherical aberration due to the objective lens as low as possible, thereby increasing the aberration margin of the optical pickup system as a whole; there is a strong need for an optical pickup objective lens capable of doing this.
When attempting to achieve the required properties in small, high-performance lenses for image pickup systems, the error permitted in manufacturing is as low as in the case of optical pickup lenses. In small, high-performance image pickup lenses, sensitivity to error in eccentricity and inclination between upper and lower surfaces increases, and to the extent that coma aberration increases, it is necessary to reduce spherical aberration. There is a need for a lens for image pickup systems that has low spherical aberration.
Accordingly, the present invention, devised in light of the above-stated technical background, has for its goal to provide a method of manufacturing even high-performance lenses by stable and continuous press molding without reprocessing of the mold even under conditions where factors causing change an the precision and properties of the lens being molded during pressing are present.
In particular, the present invention has for its objects to provide a method of manufacturing high-performance lenses in which lenses having a difference between the actual value and the desired value of spherical aberration (third-order spherical aberration) in a high-NA single lens of within ±0.022 λrms, desirably within ±0.01 λrms, and preferably essentially zero, are stably and continuously press molded without reprocessing of the mold even under conditions where factors causing change in the precision and performance of the molded lens are generated during pressing, and to provide a lens in which the difference between the actual value and the desired value of spherical aberration (third-order spherical aberration) is within ±0.02 λrms.
The present invention focuses on the fact that even when continuously press molding a glass material under stable conditions, molding conditions are actually not constant, that is, the properties of the molded article (such as a lens) are not necessarily constant because of changes, for example, in the state of the mold separation film provided on the molding surface.
Further, by exploiting the fact that the amount of aberration occurring relative to the amount of error in the manufacturing of a high-NA single lens is large (error sensitivity is high and the error permitted in manufacturing is low), it is possible to control the precision of the third-order spherical aberration of the lens during press molding and stably mass produce high-performance lenses; the present invention was devised on this basis.
The present invention relates to a method of manufacturing a molded article by pressing a heat-softened molding material with a pair of pressing molds having molding surfaces processed to prescribed shapes (mode 1 of the manufacturing method of the present invention, hereinafter), comprising
press-molding a molding material to make a molded article,
measuring an optical property of the molded article,
correcting pressing rate of at least one of the pressing molds based on the optical property thus measured, and
further press-molding to make a molded article with the corrected pressing rate.
In this method, the correction of the pressing rate is preferably conducted based on a predetermined correlation between the pressing rate and the optical property.
The present invention further relates to a method of manufacturing a molded article by pressing a heat-softened molding material with a pair of pressing molds having molding surfaces processed to prescribed shapes (mode 2 of the manufacturing method of the present invention, hereinafter), wherein
each time a prescribed number of molded articles is press-molded, pressing rate of at least one of the pressing molds is corrected and
a molded article is further molded at the corrected pressing rate to maintain an optical property of the molded article within a prescribed range.
In this method, the correction of the pressing rate is preferably conducted based on a predetermined correlation between the number of molded articles being continuously molded and optical properties of the articles having been molded.
In the above two methods (modes 1 and 2), the optical property may be spherical aberration.
The present invention still further relates to a method of manufacturing a molded article by pressing a heat-softened molding material with a pair of pressing molds having molding surfaces processed to prescribed shapes (mode 3 of the manufacturing method of the present invention, hereinafter), comprising
press-molding a molding material to make a molded article,
measuring a shape of the molded article,
correcting pressing rate of at least one of the pressing molds based on the shape thus measured, and
further press-molding to make a molded article with the corrected pressing rate.
In this method, the correction of the pressing rate is preferably conducted based on a predetermined correlation between the pressing rate and the shape, said shape being the paraxial radius of curvature of either a first surface or a second surface of the molded article.
The present invention also relates to a pressing device comprising a pair of pressing molds having molding surfaces processed to prescribed shape, and a means of driving one of the pair of pressing molds at a prescribed rate to press mold a molding material supplied between the pressing molds, wherein the device farther comprises a means for detecting an optical property, a shape or a number of the molded articles and a means for controlling driving of said means of driving by correcting pressing rate of the molds based on the detected property, shape or number.
The present invention further relates to an objective lens for optical picking up, having a numerical aperture of greater than or equal to 0.6, a paraxial radius of curvature of less than or equal to 3 mm, an effective lens diameter of greater than or equal to 5 mm, and a maximum surface inclination of greater than or equal to 45 degrees with a third-order spherical aberration of within ±0.02 λrms at a prescribed wavelength (λ) of less than or equal to 430 nm; and
an objective lens for optical picking up, having a numerical aperture of greater than or equal to 0.6, a paraxial radius of curvature of less than or equal to 3 mm, an effective lens diameter of greater than or equal to 5 mm, and a maximum surface inclination of greater than or equal to 45 degrees with a wave front aberration of less than or equal to 0.04 λrms at a prescribed wavelength (λ) of less than or equal to 430 nm.